Dust Devil Horizontal Velocities and Directions of Motion on Mars Derived from Crism and Ctx/Hirise Observations

44th Lunar and Planetary Science Conference (2013) 2141.pdf

DUST DEVIL HORIZONTAL VELOCITIES AND DIRECTIONS OF MOTION ON MARS DERIVED FROM CRISM AND CTX/HIRISE OBSERVATIONS. D. Reiss1, A. Spiga2 and G. Erkeling1, 1Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany, 2Laboratoire de Météorologie Dynamique, Université Pierre et Marie Curie, Paris, France.

Introduction: Early field observations suggested that line the exact spacecraft clock time is recorded in tabular dust devils travel across the surface with horizontal veloc- files, hence the time elapsed after the image start time can ities and in directions which, to first order, correspond to be calculated. Imaging times of the equivalent dust devils ambient wind fields (e.g., 1-3). The more recent and de- in CTX or HiRISE images were read out in the ISIS im- tailed field measurements of [4] showed that dust devil age viewer QView after ISIS3 processing. We did a sys- ground velocity is in agreement with boundary layer tematic dust devil survey based on all available CRISM winds a few tens of meters above the surface, and that center targeted (FRT, HRS, and HRL) VNIR browse im- dust devil direction closely matches ambient wind direc- ages available at PDS Geoscience Node released until 01 tions. These results suggest that the horizontal motion of October 2012. terrestrial dust devils can be used as a proxy for ambient Results: In total, 47 dust devils were identified in 26 wind speeds and directions a few tens of meters above the CRISM images which have counterparts in CTX and/or surface [4]. As pointed out by those authors, there should HiRISE. For 44 of the dust devils we were able to meas- be no physical reason why this relationship would differ ure their horizontal speeds. Figure 1 shows one example on Mars. Preliminary tests using a few measurements from Gusev crater. Horizontal speeds of dust devils range indicated indeed a broad agreement between dust devil between ~4 to ~25 ms-1 with an average speed of ~12 ms- horizontal velocities [5] and directions of motion [6] with 1. For more than two-third (73%) of the dust devils the ambient wind speeds and directions predicted at the horizontal speed was less than 15 ms-1. Measured hori- height of the measured dust devils through Global Cli- zontal speeds are in good agreement with previous meas- mate Models [GCMs]. urements on Mars derived from HRSC data [6,10]. Here, we introduce a new technique to derive horizontal speeds and directions of motion for martian dust devils using image data from the Compact Reconnaissance Im- aging Spectrometer for Mars (CRISM) [7] together with time delayed surface observations by the CTX and/or HiRISE. This is the first time this combination of instruments on board MRO is used to retrieve the characteris- tics of dust devils. We compare our dust devil horizontal speed and direction of motion to the monthly climatologies released in the Mars Climate Database (MCD) [8,9] derived from General Circulation Model (GCM) predictions. Method: While the CTX and HiRISE (red channel) Figure 1. Example of a coordinated CRISM and HiRISE observations at Gusev crater. (A) CRISM image instruments image the same spot on the martian surface at FRT0001D8DC_07_IF124S with a ~50 m in diameter dust devil. nearly identical times, CRISM uses a specific imaging (B) The same dust devil as observed in A imaged by HiRISE technique which results in larger time offsets to the other (ESP_021925_1650) ~34.2 s later. The dust devil moved ~415 m imaging instruments onboard MRO. The CRISM instru- between the CRISM and HiRISE observation indicating a horizontal -1 ment uses an active pointing device, a gimbaled sensor speed of ~12.1 ± 1.6 ms . The dust devil moved in southeast direc- system enabling that a target can be tracked with long tion. exposure time. During an observation the center of a sur- We compared our measured dust devil horizontal veloci- face target is imaged in forward looking, nadir, and ties and directions of motion to the monthly climatologies backward looking angles in flight direction commanded released in the MCD derived from GCM predictions. In by the Gimbal Motor Electronics (GME) unit (for further general, the dust devil horizontal velocities and directions details see [7]). This imaging technique results in positive of motion are in good agreement with the predicted MCD and negative time offsets (± 1 minute) from the center of horizontal wind speeds at the top of the dust devil heights the surface target to the simultaneously in flight direction (Fig. 2) and MCD horizontal wind orientations (Fig. 3). scanning HiRISE and CTX instruments. We calculated However, some dust devil horizontal velocities (id 28-30, the exact acquisition time of the center of dust devils in and 32 in Fig. 3) and directions of motion (id 28-30 in CRISM raw images using the start and stop time of the Fig. 4) show a large offset to the MCD wind speed and image acquisition in combination with the image line wind orientation predictions at times the dust devils oc- position of the dust devil center. For each CRISM image curred. Fortunately, these dust devils occurred near the 44th Lunar and Planetary Science Conference (2013) 2141.pdf

Phoenix landing site (id 28-30) and at Gusev crater (id the observation time of CRISM/CTX on 2008-10-16 the 32) which allows us to compare the offsets with Phoenix lander was still active which allows us to com- lander/rover observations. pare our results with lander observations. The three dust 30 devils observed by CRISM and CTX occurred at LS = 142.83° and moved with a horizontal velocity of ~15 ms-1 32 25 in eastward direction. This is in contradiction to the MCD predictions with a horizontal wind speed of ~2 ms-1 and a

20 35 wind orientation to the west. However our results are 3 30 45 consistent with the landing site observations at the time of 36 29 the dust devil occurrence. Wind measurements at the 15 28 landing site show relatively high wind speeds in eastward direction [12]. Based on mapping of dust devil tracks and 10 wind streaks from orbital datasets, [12] found a predomi- nant wind direction of WNW or ESE and suggested that Dust devil observed horizontal speed [ms-1] speed horizontal observed Dust devil 5 dust devils move more likely in eastward direction due to the wind direction measurements after LS = 120°. In addi- tion, [13] surveyed the Surface Stereo Imager (SSI) da- 0 0 2 4 6 8 10 12 14 16 taset mounted on the Phoenix lander for active dust devil MCD horizontal wind speed [ms-1] passages. A total of 76 active dust devils were detected Figure 2. Dust devil horizontal velocities versus MCD horizontal during LS = 125° - 142° [13]. Although it was difficult to speeds at the top of the dust devil heights. determine the exact travelling direction of these dust devils due to the lack of clear landmarks the most likely direction of motion was suggested to be toward the east [13]. Our measurements show that all three dust devils move toward the east supporting the suggested travel directions of [12] and [13]. Orbital data show evidence for passing of weather systems consisting of condensate clouds over the Phoenix landing site after LS = 111° [12- 14] suggesting that regional weather phenomena are re- sponsible for our offsets between dust devil observations and predicted model results.

30 Conclusions: Our results show that dust devils on

19 29 Mars move with ambient wind fields confirming previous 17 28 16 studies on Mars [5,6] and Earth [4]. They also indicate that dust devils on Mars move with a horizontal speed which equals boundary layer wind speeds, hence faster than near surface winds in agreement with terrestrial results [4]. We introduced new method for measuring hori- Figure 3. Dust devil directions of motion versus MCD horizontal zontal speeds and directions of motion of larger scale wind orientations. active features on Mars (e.g., dust devils, dust storms). Gusev crater: The horizontal velocity of dust devil 32 is The combination of this method with simultaneous meas- -1 with ~25.5 ± 4.7 ms much higher than the predicted urements like dust devil tangential velocity derivations -1 ambient wind speed with ~10 ± 1 ms although the direc- from HiRISE color images [15] and/or current landing tion of motion is in very good agreement with the predict- site dust devil observations in Gale crater would lead to a ed MCD wind orientation. A large dataset of dust devil much better understanding of dust devil processes on horizontal velocities was derived from MER-A (Spirit) Mars. image sequences at Gusev crater [11]. The horizontal References: [1] Wegener A. (1914) Meteorologische velocities range between ~0.1 and ~27 ms-1 (n=498), but Zeitschrift 31, 199–200, [2] Flower W.D. (1936) London Meteorol. the median speeds around 2 ms-1 are relatively low [11]. Off. Prof. Notes. 5, 1–16. [3] Crozier W.D (1970) JGR 75, 4583– Although, most of the horizontal velocities measured by 4585. [4] Balme M.R. et al. (2012) Icarus 221, 632-645. [5] Stanzel [11] are much lower, it shows that our dust devil horizon- C. et al. (2006) GRL 33, L11202. [6] Stanzel C. et al. (2008) Icarus 197, 39-51. [7] Murchie S.et al. (2007) JGR 112, E05S03. [8] Lewis tal velocity is not out of range. Interestingly, another dust S.R et al. (1999) JGR 104, 24177–24194. [9] Millour E. (2008) LPI devil in Gusev crater was observed (Fig. 1) which shows -1 Contribution No. 1447, Abstract #9029. [10] Reiss D. et al. (2011) a horizontal velocity of ~12.1 ± 1.6 ms and a direction Icarus 215, 358–369. [11] Greeley R. et al. (2010) JGR 115, of motion toward southeast, both in very good agreement E00F02. [12] Holstein-Rathlou C. (2010) JGR 115, E00E18. [13] with the MCD model predictions. Ellehoj M.D. (2010) JGR 115, E00E16. [14] Moores J.E. (2010) Phoenix landing site: Three active dust devils were ob- JGR 115, E00E08. [15] Choi D. S. and Dundas C.M. (2011) GRL served about 2.5 km north of the Phoenix landing site. At 38, L24206.